Girls of all races are entering puberty earlier than ever before, and new research suggests that obesity may a contributing factor, particularly in white girls.
Early puberty has been linked to a number of medical conditions, including increased risk of breast and ovarian cancer, as well as high blood pressure and depression.
White girls on average began developing breasts at a median age of 9.7 years, about four months earlier than found in a separate study in 1997, said the report in the journal Pediatrics.
A higher body mass index, or ratio of height and weight, was "the strongest predictor" of early breast development across all races in the study.
African-American girls continued to develop at earlier ages than whites and Asians, as previous studies have also found, though no earlier than was observed in previous studies from the late 1980s and 1990s.
"The obesity epidemic appears to be a prime driver in the decrease in age at onset of breast development in contemporary girls," said the research led by Frank Biro at Cincinnati Children's Hospital.
The research is based on a group of 1200 girls in New York, the San Francisco Bay area and the greater Cincinnati area, as part of the government funded Breast Cancer and the Environment Research Program.
The girls enrolled in the study beginning in 2004 when they were between the ages of six to eight, and their development was tracked for multiple years.
A trend toward earlier puberty has been noted in past international studies as well.
Research from Denmark published in 2009 showed that girls were experiencing breast development nearly a year earlier than those born 15 to 16 years before.
An accompanying editorial in Pediatrics by Marcia Herman-Giddens of the University of North Carolina at Chapel Hill said that the reasons for early puberty are complex and may not be fully understood for some time.
"Each individual girl is exposed to multiple factors in today's environment, many not present decades ago, that may potentially influence her pubertal onset," she wrote.
"Furthermore, because early puberty and menarche are associated with many detrimental health and psychosocial issues, we must not accept this premature development as the 'new normal'."
Instead, she called for a renewed push to create "a healthier environment for our children".